Actinide and/or transuranic material destruction occurs naturally along well-understood decay chains to eventually become a stable non-radioactive element, lead. However, in natural decay, some of these elements remain dangerous to man over hundreds of thousands or millions of years.
Actinides are separated from spent nuclear fuel, and other components containing fissile materials, by chemical means. Fission and actinide products are left over from the splitting of atoms to make power and are the principal residual material in spent nuclear fuel. The fission products are short-lived by comparison to the actinide products, and are destroyed by the natural radioactive decay process in about one thousand years. Most actinide products, on the other hand, remain dangerous for many centuries. It is this extremely long decay process that results in target isolation objectives of the US Department of Energy (DOE) of 1,000,000 years.
In destruction by the natural decay process, the radioactive elements arrive at a stable state by the spontaneous emission of radioactive particles, including alpha particles and neutrons. By increasing the rate at which actinides decay to a stable state, the technological challenge and cost burden of establishing long-lived containment systems for the disposal of spent fuel may be significantly reduced.
The potential benefits of effectively reducing the half life of actinide elements has resulted in extensive experimental and operational programs having the sole objective of hastening the transmutation and decay of these products to a stable, and inherently safe, state.
Most applications directed to the destruction of actinides by transmutation or fission rely upon high neutron flux rates, similar to those found in operating reactors. To that end, many applications propose the inclusion of waste actinides in some portion of an active fuel assembly so that the material is transmuted or destroyed using the high neutron flux of the operating reactor.
While these methods do accelerate transmutation of the actinides to more stable forms thereby reducing the actinide waste quantity requiring disposal, they have the disadvantage of requiring special packaging, high-energy neutron sources and handling of the waste forms, which increases personnel radiation exposure and the risk of accident. The nuclear reactor or particle accelerator operations required by these methods are both complex and expensive.
This invention operates by converting the abundant alpha particles emitted by the actinides into neutrons via an alpha—neutron (alpha, n) reaction that is a property of Beryllium and some other elements such as oxygen. This raises the neutron flux of the container to about one ten thousandth of the level present in a nuclear reactor intended to burn the actinides in one or two years. The invention makes use of its passive nature and the 10,000-year minimum period required by regulations to accomplish the same level of destruction of the actinide waste after emplacement in a geological repository. By utilizing the invention, a million year waste isolation period is no longer required, and the shorter 10,000-year waste isolation period is much less complex to analyze and regulate.
As described herein, the preferred embodiment of the invention is intended for use with the current series of DOE Standard Canisters (herein after “Canisters” or “Canister”) designed for geologic disposal.